Module 5 Photosynthesis

Question 1

Respiration and Photosynthesis

Answer 1

• Photosynthesis occurs in photosynthetic organisms and stores energy as light is converted to chemical energy in simple organic compounds. It is an anabolic reaction as carbon dioxide and water form glucose and oxygen. ATP is produced in photophosphorylation, in the chloroplasts, requiring carbon dioxide and light. There is an electron transport chain, and a coenzyme NADP is used.
• Aerobic respiration occurs in all living organisms. Chemical energy is released, by converting simple organic compounds into a more accessible form of energy. The reaction is catabolic as glucose and oxygen are broken down to carbon dioxide and water. The reaction occurs in the mitochondria, using the coenzyme NAD, with oxygen required but not light. ATP is produced by substrate level and oxidative phosphorylation. There is an electron transport chain.
• Anaerobic respiration takes place as lactate fermentation in the muscle cells of animals and ethanol fermentation in plants and fungi. It is a catabolic reaction as glucose is broken down to ethanol or lactate, releasing energy. ATP is produced by substrate level phosphorylation in the cytoplasm. No light or oxygen is required. The coenzyme NAD is used.

Question 2

Chloroplasts

Answer 2

• Chloroplasts are the site of photosynthesis and larger than mitochondria at 2-10 micrometres.
• They are surrounded by a double membrane envelope. The outer membrane is permeable to small molecules and ions. The inner membrane contains transport proteins that control what enters and leaves the chloroplast. The stroma is a cytosol-like fluid with CO2, enzymes and sugars dissolved in it. A system of flat, fluid filled sacks called thylakoids contain enzymes, pigments and electron carriers. Stacks of thylakoids called grana are connected by lamellae. Pigment molecules are arranged in light harvesting systems called photosystems, with a funnel like structure directing towards the primary pigment.
• They also contain 70S ribosomes, a loop of DNA that codes for chloroplast proteins and starch grains, which are stores of sugars formed in photosynthesis.
• The light dependent stage takes place on the thylakoid membrane and the light independent stage takes place in the stroma.
• Chloroplasts are adapted for photosynthesis. The stroma contains enzymes that catalyse the light independent reaction and surrounds the grana for rapid transport of the products of the light dependent stage. The grana are stacked to increase surface area, so the maximum amount of light is absorbed for photosynthesis, and there is sufficient space for electron carriers and ATP synthase. Chloroplast DNA codes for enzymes and proteins used in photosynthesis and ribosomes carry out the synthesis of these proteins. The inner membrane contains selective transport proteins, that control the flow of molecules between the stroma and cytosol.

Question 3

Photosynthetic Pigments

Answer 3

• Different photosynthetic pigments absorb different wavelengths of light. The two types of pigment are carotenoids and chlorophyll. Chlorophyll A is blue-green, B is yellow-green, beta carotene is orange, and xanthophyll is yellow. Chlorophylls absorb blue-violet and red light; carotenoids absorb blue-violet light. Green light is reflected.
• Pigment molecules are arranged in light harvesting systems called photosystems. These are funnel like structures directing towards the primary pigment, chlorophyll a. Accessory pigments absorb different wavelengths to the primary pigment and so increase the range of wavelengths of light that can be absorbed.
• Photosystem 1 contains chlorophyll a with a maximum absorbance of 700. Photosystem 2 contains chlorophyll a with a maximum absorbance of 680.
• Chromatography can be used to separate different pigments as they travel at different speeds due to size and charge. Pigments can be determined by calculating rf values. It does not show how much pigment is present.

Question 4

Light Independent Stage

Answer 4

• In the light independent stage, the Calvin cycle produces carbohydrates. Energy from light is not required, but the products from the light dependent reaction are, so the reaction cannot continue indefinitely in the dark.
• Rubisco first catalyses the fixation of carbon dioxide in a reaction with RuBP, forming an unstable 6 carbon molecule, which splits into two molecules of 3 carbon GP. Energy from ATP and hydrogen from reduced NADP is then used to reduce GP to two molecules of TP. In the formation of 2 molecules of TP, 2 ATP and 2 reduced NADP are used. 1/6th of the TP formed is then used to form carbohydrates, while 5/6th is used to reform RuBP, requiring 1 molecule of ATP.
• TP can be used to form hexose phosphates to form carbohydrates like starch, sucrose and cellulose. It can also be converted to glycerol and fatty acids to form lipids, or amino acids to form proteins.

Question 5

Factors affecting photosynthesis

Answer 5

• If there is a shortage of CO2, photosynthetic pigments, water, light intensity or temperature, photosynthesis will not take place at the maximum rate.
• The main limiting factors are carbon dioxide concentration, light intensity and temperature.
• If light intensity increases, the rate of photosynthesis will also increase. The light dependent stage will happen at a faster rate, meaning more ATP and reduced NADP are produced for the Calvin cycle. When the line plateaus, it is no longer a limiting factor, and another factor is limiting rate. If light intensity decreases, the amount of GP will increase, but the amount of TP and RuBP will decrease. Less ATP and reduced NADP is formed in the LDS, so less GP can be reduced to TP, and RuBP is not formed.
• If the concentration of carbon dioxide increases, the rate of photosynthesis increases. More carbon dioxide can react with RuBP to form 2GP, at a faster rate. Too high levels of carbon dioxide can become toxic. If carbon dioxide concentration decreases, the amount of RuBP will increase and the amounts of GP and TP will decrease. More RuBP remains unfixed, and less GP and TP is formed.
• If the temperature increases, the rate of photosynthesis will increase, as the reaction is controlled by enzymes. It will affect the LDR less, as this is controlled by energy from light instead of the kinetic energy of reacting particles. The Calvin cycle is controlled by enzymes like rubisco, so increasing the temperature will increase the rate of reaction. If temperature increases, stomata will close to reduce water loss, which prevents carbon dioxide from entering. Too high temperatures will also affect membrane permeability and chemiosmosis and cause enzymes to denature.
• Yield can be increased using glasshouses that control these factors. This can however be costly and have environmental implications.
• Practically, these factors can be investigated. Light intensity can be measured by changing the distance from a light source. Temperature can be changed using a water bath, and carbon dioxide concentration by changing the concentration of sodium hydrogen carbonate. The volume of oxygen produced is measured.
• The rate of the LDR can be measured using a redox indicator, that acts as an electron acceptor instead of NADP. The rate of LDR is measured by the time taken for the indicator, such as DCPIP to change colour, as it is reduced.